I think he did a good job, you're pretty much center. Better then my 50 year old mother at horse shows--I'm lucky if she gets the right horse and rider. ;)
Your shoulder will rotate forward because the one had is in front and the other hangs back. It's hard to keep them square--everyone does it.
Sitting on your lower back may make your back sore... it takes the bend out of the spine, and the bend is what keeps your back like a 'spring'.
If he's sloppy, he's saying, 'mum, I don't have muscle for this yet'. Push him forwards until the lope feels comfortable and not hard to keep together, and then do lots of transitions, lope-to-trot (not jog). When he changes, let him drop from the lope to a nice, forward going trot. You don't want him to slam on the brakes, you want him loping, and then IMMEDIATE ground-covering trot. When you start doing this in quick succession and he's not nervous, (four strides canter, four strides trot, four strides canter, etc. etc.) you'll feel him lift his back. And you'll watch him drop his neck. :) Forward means calm and ground covering though--not fast, nervous, or choppy.
If you want, you can do this in english tack so that he can build the muscles, but not associate 'fast and forward' with western stuff. When he can do these things comfortably, I'd even introduce lateral work. Then put him in your western tack, and ask him for something slow, but with energy. You'll be surprised how he'll keep it together when you prepare his muscles to do so. ;)
With the forward/lateral work, you'll also see his hind end get round, and less angular. Correct work= correct muscles.
Here's a decent picture of my gelding showing a nice, forward trot, reaching into the bit and working those muscles he'll later be using for WP-